Extrusion and injection molding of plastic and other materials has become increasingly important in the last forty years or so. In the process, extrusion screws force a molding material through specially shaped dies to produce a wide variety of products. The screws are usually housed in steel cylinders.
The basic process and equipment for carrying out the process were refined in a number of ways in the early stages of development but one problem which has still not been completely overcome is wear and corrosion of the steel cylinders, and the problem has become more pronounced in recent years with the advent of new fillers for plastics. Many of such fillers are themselves abrasive and contribute to the wear of the cylinders and the screws.
A promising contribution to the development of a cure for the excessive wear problem is the bimetallic cylinder. Kormann and Hirsch were pioneers in this development in the early 1930's and some of their work is described in U.S. Pat. Nos. 2,049,913 and 2,046,914, each issued on July 7, 1936. The process described in these patents includes placing the steel cylinder in a horizontal position and loading the cylinder with a preselected quantity of an alloy having a melting point less than the melting point of the cylinder itself.
The ends of the cylinder are then plugged by welding caps over them and the cylinder is gradually heated to the melting point of the lining alloy. The cylinder is rapidly spun about its axis to centrifugally spread the melted lining alloy over the inner surface of the cylinder. After the cylinder is cooled, the end caps are removed and lathes and hones are used to finish the inside surface of the lining to the desired diameter and smoothness.
Kormann and Hirsch employed ferrous alloys as their cylinder lining, e.g. the one described in U.S. Pat. No. 2,046,913. This alloy includes carbon from about 21/2 - 31/2%, boron 0.75 - 11/2%, nickel 21/2 - 6%, less than 11/2% silicon and trace amounts of sulphur and phosphorus in addition to iron. The percentages are weight percentages.
Another ferrous alloy developed for use in bimetallic cylinders is described in Saltzman U.S. Pat. No. 3,658,515 issued Apr. 25, 1972. This alloy preferably includes from 3.3 - 3.9% carbon, 0.75 - 1.25% boron, 1.2 - 1.6% manganese, 0.65 - 1.10% silicon, 4.1 - 5.0 percent nickel, 0.9 - 1.4% chromium, up to 0.5% molybdenum and the balance iron.
While these ferrous alloys have hardnesses in the approximate range of 58-64 Rockwell C in their centrifugally cast state and possess good wear resistance to abrasive plastic fillers, the ferrous materials exhibit poor corrosion resistance. In fact, the high ferrous content under corrosion conditions has the effect of causing some plastics to decompose under extrusion conditions. Some ferrous alloys were developed which had improved corrosion properties, but those alloys had lower hardness characteristics.
To overcome these new problems, a series of non-ferrous bimetallic cylinder lining alloys have been developed. These typically include about 40% nickel, 45% cobalt, 8% chromium and 3% boron in addition to minor amounts of carbon, manganese, silicon, boron. While these alloys have superior corrosion resistance, they do not have wear and abrasion characteristics required for the bimetallic cylinder apparatus.
A very recent modification of the non-ferrous alloys comprises mixing tungstin carbide particles with the non-ferrous material at the time it is inserted into cylinder. A typical composition includes 30 - 40% tungsten carbide, 22 - 61% nickel, up to 37% cobalt, up to 12% chromium 1.3 - 3.0% boron and smaller amounts of iron, silicon, manganese and carbon. The tungsten carbide alloy when centrifugally cast includes higher concentrations of discrete tungsten carbide particles adjacent the interface between the lining and the steel backing, and the inner surface of the lining is relatively easy to finish and has fairly good abrasion and corrosion properties. However, because the tungsten carbide concentration varies throughout the thickness of the lining, the cylinders are subject to uneven rates of wear during use and the discrete tungsten carbide particles have a sandpaper-like effect on the extrusion screws.